1. Field of the Invention
The present invention relates to an optical recording medium readout apparatus in which read beam power is controlled based on the amplitude ratio of a readout signal from a magnetic super resolution optical recording medium.
2. Description of the Related Art
Magnetic super resolution magneto-optical readout art has been developed that in magneto-optical disks representing optical recording media provided with a readout layer possessing in-plane magnetization and a recording layer, readout of recorded marks smaller in diameter than the spot size of a beam of light is permitted by as a result of irradiation with a beam of light directed from a readout layer of an magneto-optical disk, causing transfer of magnetic state at the recording layer in correspondence to magnetic state at the readout layer, with transition from in-plane magnetization to perpendicular magnetization, in a portion (hereinafter xe2x80x9caperturexe2x80x9d) of the irradiated region at which the temperature rises above a predetermined temperature.
In such art, due to changes in ambient temperature during readout it is possible for the optimum readout power of the beam to fluctuate despite the fact that the drive current causing generation of the beam is held constant. Furthermore, when readout power is too high, this causes aperture size to become too large, increasing the presence of crosstalk from adjacent tracks in the output readout signal, decreasing the signal-to-noise ratio of the data being read, and increasing the frequency of occurrence of read errors. Moreover, when readout power is too low, this can cause aperture size to become smaller than the size of recorded marks, and can cause the output of the readout signal from the track being read to become too weak, increasing the frequency of occurrence of read errors.
As a remedy to the foregoing problem, in the conventional art as disclosed at Japanese Unexamined Patent Publication JP-A 8-63817 (1996), two species of readout power control marks having different lengths are provided on a magneto-optical disk, readout power being constantly maintained at an optimum value and frequency of occurrence of read errors being held to an acceptable level as a result of reading these marks and controlling readout power so as to cause the ratio between amplitudes of the signals read from those marks to approach a predetermined value.
Furthermore, in the conventional art as disclosed at Japanese Unexamined Patent Publication JP-A 2000-99945 (2000), in the event that normal detection of amplitude ratio from the signals read from the foregoing readout power control marks is for some reason interrupted, by holding readout power at the value it had immediately preceding occurrence of the inability to carry out normal detection it is possible to prevent abnormality in the controlled readout power.
FIG. 11 is a block diagram showing the constitution of an optical recording medium readout apparatus 1 in the conventional art. FIG. 12 is a drawing showing the structure of a magneto-optical disk 10 representing an optical recording medium. A first sector 15 at a track on an information recording surface of the magneto-optical disk 10 comprises an address region 15a indicating the location of the sector, a readout power control region 15b in which a repeating pattern of short and long marks representing readout power control marks is recorded, and a data recording region 15c in which digital data is recorded. At a track on an information recording surface of the magneto-optical disk 10 other than the track containing the first sector 15, there is a second sector 16, and the second sector 16 comprises regions similar to those of the first sector 15.
The optical recording medium readout apparatus 1 comprises an optical head 2, an amplitude ratio detection circuit 3, a differential amplifier 4, a readout power control circuit 5, a switch 6, a readout power value storage circuit 7, a seek status detection circuit 8, and a data readout circuit 9. The optical head 2 comprises a semiconductor laser source 2a and a photodiode 2b. Laser light from a light beam emitted by the semiconductor laser source 2a is incident at the address region 15a of sector 15 of the optical disk 10 and is reflected therefrom. The laser light reflected at the address region 15a is incident on the photodiode 2b, where it undergoes photoelectric conversion, allowing the optical recording medium readout apparatus 1 to identify the sector address corresponding to the location of sector 15.
Furthermore, laser light emitted from the semiconductor laser source 2a is incident at the readout power control region 15b of sector 15 and is reflected therefrom. The laser light reflected at the readout power control region 15b, now containing information about a repeating pattern of short and long marks thereat, is incident on the photodiode 2b, where it undergoes photoelectric conversion to become a control readout signal. The control readout signal is input at the amplitude ratio detection circuit 3, where an average amplitude ratio is calculated. The average amplitude ratio and a target value for amplitude ratio are input at the differential amplifier 4, where a value is calculated by subtracting the target value from the average amplitude ratio. The readout power control circuit 5 outputs a signal indicating a value corresponding to an optimum readout power based on which the semiconductor laser source 2a can be controlled so as to cause the value calculated by the differential amplifier 4 to go to zero. The signal indicating a value corresponding to an optimum readout power which is output from the readout power control circuit 5 is input at the switch 6, and is also input at the readout power value storage circuit 7, where it is stored.
In accordance with a signal from the seek status detection circuit 8 which is connected to the optical head 2, the switch 6 causes either the readout power control circuit 5 or the readout power value storage circuit 7 to be electrically connected to the semiconductor laser source 2a, causing a drive current corresponding to the readout power value to be delivered to the semiconductor laser source 2a. The seek status detection circuit 8 detects the status of movement(hereinafter xe2x80x9cseekxe2x80x9d) of the optical head 2 between tracks. When the seek status detection circuit 8 sends a detection signal to the switch 6 indicating that the optical head 2 is not in mid-seek, in accordance with such detection signal the switch 6 causes the readout power control circuit 5 to be electrically connected to the semiconductor laser source 2a. The semiconductor laser source 2a is thereafter driven with a drive current such as will produce an optimum readout power value as indicated by the signal output from the readout power control circuit 5, emitting laser light, and the laser light is incident at the data recording region 15c of sector 15, is reflected therefrom, and is incident on the photodiode 2b, where it undergoes photoelectric conversion to become a readout signal, and this is input at the data readout circuit 9. The foregoing sequence of events is repeated for sectors following sector 15 within the same track as sector 15, with the optimum readout power being reset to a new value for each sector. This allows readout information data to be output with a low error rate.
When the seek status detection circuit 8 delivers a detection signal to the switch 6 indicating that the optical head 2 has initiated a seek operation which will move it from the track containing sector 15 to the track containing sector 16, in accordance with such detection signal the switch 6 causes the readout power value storage circuit 7 to be electrically connected to the semiconductor laser source 2a. At this time, the semiconductor laser source 2a is supplied with a drive current such as will produce a readout power of value as indicated by the signal output from the readout power value storage circuit 7, which value is stored at the readout power value storage circuit 7, causing emission of laser light from the semiconductor laser source 2a. The readout power value stored at the readout power value storage circuit 7 is the value of the optimum readout power as determined for the sector immediately preceding the start of the seek operation; i.e., sector 15. Control based on amplitude ratio is temporarily suspended at such a time, with the readout power value delivered to the semiconductor laser source 2a being held fixed instead. Thereafter, when the seek operation is completed and the optical head 2 has finished moving to the track containing sector 16, the seek status detection circuit 8 sends a detection signal to the switch 6 indicating that the optical head 2 is not in mid-seek, upon which the switch 6 causes the readout power control circuit 5 to be electrically connected to the semiconductor laser source 2a. Thereafter, operations are as described above.
By thus providing each sector with a readout power control region, this being a region for recording marks for control of readout power, and detecting control readout signals for control of readout power at each sector, it is possible to carry out readout power control such that response occurs with short period and to track fluctuations in optimum readout power value with small lag time.
In the optical recording medium readout apparatus 1, the value of the readout power of the semiconductor laser source 2a when reading the readout power control region of sector 16 is held fixed at the readout power value which is stored at the readout power value storage circuit 7 at the time of the start of the seek operation to move to sector 16, i.e., at the time that control of readout power based on amplitude ratio is temporarily suspended. Because of the mutually different radial positions of the track containing sector 15, this being the location of the optical head 2 when readout power control is suspended, and the track containing sector 16, it is possible for there to be significant difference in tilt therebetween, this tilt being inclination due to warpage of the magneto-optical disk 10, runout and/or inclination of the shaft of the motor causing rotation of the magneto-optical disk 10, and so forth. While a change in tilt will, because of the resulting change in effective power, effective power being the actual readout power of the beam at the information recording surface of the magneto-optical disk 10, produce a change in the readout power necessary to produce an aperture of optimum size, i.e., in optimum readout power, because aperture size more or less corresponds to amplitude ratio and because readout power is being controlled so as to produce an amplitude ratio approaching a predetermined value, readout power following the seek operation, when the optical head 2 is at sector 16, will tend to approach an optimum readout power reflecting the amount of tilt at sector 16.
FIG. 13 is a graph showing results of actual measurements indicating the relationship between readout power and optimum amplitude ratio V2T/V8T when there is practically no tilt and when there is a large amount of tilt. Amplitude V2T is the value of an amplitude during detection of the short marks (hereinafter xe2x80x9cshort marksxe2x80x9d) present in the repeating pattern of short and long readout power control marks recorded in the readout power control region on the magneto-optical disk 10, and amplitude V8T is the value of an amplitude during detection of the long marks (hereinafter xe2x80x9clong marksxe2x80x9d) present in the repeating pattern of short and long readout power control marks recorded in the readout power control region. Because optimum amplitude ratio V2T/V8T is on the order of 0.59 both when the optimum readout power at a time when there is practically no tilt, i.e., the readout power at a time when the error rate is at its lowest value, is 1.5 mW, and when optimum readout power at a time when there is a large amount of tilt is 1.7 mW, carrying out control of readout power so as to cause the amplitude ratio which is detected to always approach 0.59 will permit optimum readout power to always be maintained despite any variation in the amount of tilt.
Now, imagine that at the time of the start of a seek operation the readout power value is 1.45 mW due to error in the amplitude ratio detected when there is practically no tilt, this being smaller than the optimum readout power value of 1.5 mW. Starting a seek operation under such circumstances will cause readout power to be held at the value which is stored in the readout power value storage circuit 7, or 1.45 mW. Upon completion of the seek operation, while control of readout power based on amplitude ratio resumes with detection of the amplitude ratio at sector 16 with a readout power value of 1.45 mW, because of the large amount of tilt at sector 16 the amplitude ratio detected at sector 16 with a readout power value of 1.45 mW is 0.62.
Based on FIG. 13, the relationship between readout power and amplitude ratio when there is a large amount of tilt is such that amplitude ratio increases monotonically within a low readout power value domain, with amplitude ratio reaching a maximum near the point where readout power value is 1.55 mW, and amplitude ratio decreases monotonically within a high readout power value domain in which readout power value exceeds 1.55 mW. There are therefore two readout power values, 1.45 mW and 1.62 mW, which correspond to an amplitude ratio of 0.62, and the readout power control circuit 5 erroneously determines that the amplitude ratio of 0.62 which is detected at sector 16 corresponds to a readout power value not of 1.45 mW but of 1.62 mW. As a result, there is a lag in control response time because the readout power control circuit 5 attempts to alter the readout power by only an amount +0.08 mW obtained by subtracting the erroneously determined readout power value of 1.62 mW from the optimum readout power value of 1.7 mW corresponding to the target amplitude ratio, with the result that the semiconductor laser source 2a is controlled so as to deliver a readout power of 1.53 mW obtained by adding the +0.08 mW value which was obtained by subtraction above to the actual readout power value of 1.45 mW which exists at the time of completion of the seek operation, causing a lag in control response.
Furthermore, in the event that the readout power value at the time of the start of the seek operation is lower than the above value (1.45 mW), say 1.32 mW for example, starting the seek operation under such circumstances will cause readout power to be held at the value which is stored in the readout power value storage circuit 7, or 1.32 mW. Upon completion of the seek operation, while control of readout power based on amplitude ratio resumes with detection of the amplitude ratio at sector 16 with a readout power value of 1.32 mW, because of the large amount of tilt at sector 16 the amplitude ratio detected at sector 16 with a readout power value of 1.32 mW is 0.57. Based on FIG. 13, there are two readout power values, 1.32 mW and 1.75 mW, which correspond to an amplitude ratio of 0.57, and the readout power control circuit 5 erroneously determines that the amplitude ratio of 0.57 which is detected at sector 16 corresponds to a readout power value not of 1.32 mW but of 1.75 mW. As a result, not only is there is an even greater lag in control response because the readout power control circuit 5 attempts to alter the readout power by only an amount xe2x88x920.05 mW obtained by subtracting the erroneously determined readout power value of 1.75 mW from the optimum readout power value of 1.7 mW corresponding to the target amplitude ratio, with the result that the semiconductor laser source 2a is controlled so as to deliver a readout power of 1.27 mW obtained by adding the xe2x88x920.05 mW value which was obtained by subtraction above to the actual readout power value of 1.32 mW which exists at the time of completion of the seek operation, but because readout power is so low it becomes impossible to read the information recorded at the information recording surface of the magneto-optical disk 10, and in a worst-case scenario there is also the possibility that tracking servo and/or focus servo operations may become destabilized.
It is therefore an object of the invention to provide an optical recording medium readout apparatus capable of preventing abnormal control of readout power immediately following a seek operation.
The invention provides an optical recording medium readout apparatus comprising:
means for carrying out readout of information recorded on the optical recording medium by a light beam and for outputting a readout signal corresponding to readout information: and
readout power control means for controlling a readout power of a light beam so as to reach a target value therefor based on an amplitude ratio obtained from readout signals produced by reading a plurality of types of marks on an optical recording medium,
wherein the readout power control means sets readout power at a time when readout power control operations are next resumed following a temporary suspension thereof, so as to be greater than a readout power value at a point where change in amplitude ratio with respect to change in readout power goes from monotonically increasing to monotonically decreasing or from monotonically decreasing to monotonically increasing.
In accordance with the invention, because it is possible to cause readout power value at a time when readout power control operations resume to avoid abnormal power domains in which change in amplitude ratio with respect to change in readout power is not monotonically decreasing, instead entering or remaining in a normal readout power domain in which change in amplitude ratio with respect to change in readout power is monotonically decreasing, permitting readout power control operations to resume with a readout power value in the normal readout power domain, it is possible to thwart the possibility of occurrence of lag in response during readout power control operations, the possibility of divergent readout power control operations, and other such abnormal conditions.
Furthermore, in the invention it is preferable that the readout power control means includes readout power value storage means for storing a value of a readout power at a time when readout power control operations are temporarily suspended, and that the readout power control means employs as readout power value at a time when readout power control operations resume, a total value of a readout power value stored in the readout power value storage means and a predetermined value xcex1 which is not less than 0, or a value obtained by multiplying the readout power value stored in the readout power value storage means by a predetermined value xcex2 which is not less than 1.
In accordance with the invention, because it is possible to cause the readout power value at a time when readout power control operations resume to be greater than the value of a readout power at a time when readout power control operations are temporarily suspended which is stored in the readout power value storage means, it is possible with a simple constitution to avoid abnormal power domains in which change in amplitude ratio with respect to change in readout power is not monotonically decreasing, instead entering or remaining in a normal readout power domain in which change in amplitude ratio with respect to change in readout power is monotonically decreasing, and it is possible to carry out readout power control operations with high reliability.
Furthermore, in the invention it is preferable that the readout power control means includes readout power value storage means for storing a value of a readout power at a time when readout power control operations are temporarily suspended, and temperature detection means for detecting an ambient temperature, and that the readout power control means employs as readout power value at a time when readout power control operations resume, a total value of a corrected power value which is obtained by correcting a readout power value stored in the readout power value storage means based on a temperature detected by the temperature detection means and a predetermined value xcex1 which is not less than 0, or a value obtained by multiplying the corrected power value by a predetermined value xcex2 which is not less than 1.
In accordance with the invention, because it is possible to cause the readout power value at a time when readout power control operations resume to be greater than a corrected power value obtained by correcting based on a detected temperature a readout power at a time when readout power control operations are temporarily suspended which is stored in the readout power value storage means, despite the fact that an ambient temperature at a time when readout power control operations are temporarily suspended differs greatly from an ambient temperature at a time when readout power control operations resume, it is possible to carry out readout power control operations with still higher reliability.
Furthermore, in the invention it is preferable that the predetermined value xcex1 is not less than 0.2 mW, and the predetermined value xcex2 is not less than 1.2.
In accordance with the invention, causing a predetermined value xcex1 which is not less than 0 to be not less than 0.2 mW, and causing a predetermined value xcex2 which is not less than 1 to be not less than 1.2 makes it possible to cause the readout power value at a time when readout power control operations resume to be greater than the value of a readout power at a time when readout power control operations are temporarily suspended which is stored in a readout power value storage means.
Furthermore, in the invention it is preferable that the readout power control means employs as readout power value at a time when power control operations resume, a value which is on the order of a maximum allowable power value for an optical medium.
In accordance with the invention, causing the readout power value at a time when readout power control operations resume to be on the order of a maximum allowable power value for an optical medium makes it is possible to definitively avoid abnormal power domains in which change in amplitude ratio with respect to change in readout power is not monotonically decreasing, instead entering or remaining in a normal readout power domain in which change in amplitude ratio with respect to change in readout power is monotonically decreasing, making it possible to reduce to an extremely low level the possibility of divergent readout power control operations and other such abnormal conditions, making it possible to avoid accidental erasure or disruption of information recorded on the optical recording medium, and making it possible to carry out readout power control with extremely high reliability.
Furthermore, in the invention it is preferable that maximum power value read means for reading a maximum allowable power value for an optical recording medium which has been prerecorded on the optical recording medium is provided, and that the readout power control means employs as maximum allowable power value a maximum power value read by the maximum power value read means.
In accordance with the invention, because it is possible for a maximum allowable power value which has been prerecorded on an optical recording medium to be read by maximum power value read means, it is possible to carry out readout power control such that different maximum power values for optical recording media are used to carry out optimum readout power control differently for different optical recording media.